PHAROS is a unified research effort by approximately 38 centers in the Huntington Study Group in North America to study individuals at risk for Huntington's Disease who have not undergone genetic testing. It is coordinated by the Clinical Trials Coordination Center (CTCC) at the University of Rochester. The study's primary aim is to prospectively determine the phenoconversion rate in a cohort of individuals at risk for Huntington's Disease, with subjects and investigators blinded to HD gene carrier status. Secondary aims include: (1) to identify important risk factors that modify the phenoconversion rate and to compare these prospectively derived data with those of retrospective studies; (2) to assess inter-rater reliability in the determination of phenoconversion; (3) to determine the false-positive rate of phenoconversion; and (4) to assesss the ethics and feasibility of conducting a study of asymptomatic at risk individuals in a blinded setting with strict confidentiality. The study aims to enroll approximately 1000 asymptomatic subjects between the ages of 30 and 54 years old who are at risk by virtue of having had a parent or sibling with HD. Subjects will undergo double-blinded DNA testing for the trinucleotide expansion, and undergo clinical assessments every nine months for a minimum of three years.